Cooling structures find use in a variety of applications. One class of cooling structures utilizes the compression, translation, and subsequent expansion of a gas to provide cooling effects.
FIGS. 1-1D show simplified cross-sectional views of a conventional Stirling cryocooler apparatus. FIG. 1 shows the basic Stirling cooler structure 1, wherein tube 2 contains a compressible gas 4 positioned between two moveable pistons 6 and 8. A first heat exchanger structure 10 is positioned in contact with the gas proximate to first piston 6. A second heat exchanger structure 12 is positioned in contact with the gas proximate to second piston 8. A thermal regenerator 14 in contact with the gas is positioned between the first and second heat exchangers 10 and 12.
Operation of the Stirling cooler shown in FIG. 1 is now described in connection with FIGS. 1A-1D. Generally, first piston 6 serves as a source of a pressure oscillation, and second piston 8 offers resistance to the pressure oscillation created by the first piston.
Specifically, in FIG. 1A, work is applied from an external source to move first piston 6. As shown in FIG. 1B, compressible gas 4 within tube 2 responds to movement of piston 6 first by being compressed, and then by being translated in the direction of the second piston 8. Some energy applied to the system at this time is absorbed and dissipated at first (hot) heat exchanger 10.
Translation of the gas compressed by the first piston is opposed by the mass of the second piston. As shown in FIG. 1C, because of the flow resistance posed by the second piston, translation of the gas ultimately halts and the gas expands. FIG. 1D shows that as a consequence of this gas expansion, the gas cools and second heat exchanger 12 in contact with the expanding gas absorbs thermal energy from the surrounding environment, imparting a cooling effect.
Regenerator 14 may comprise a porous solid matrix (such as parallel plates or holes, screens, felts or packed sphere beds) which intercepts heat from the gas, insulating the warm end from the cold end. As the gas flows from the warm end to the cold end, it deposits heat in the regenerator matrix, and as it flows back from cold to hot, it extracts the same amount of heat. Thus, the regenerator acts as a passive thermal insulation device.
The efficiency and effectiveness of the Stirling cooler is highly dependent upon the phase relationship between the velocity and pressure of gas within the tube. This is because the cooling mechanism requires that the gas be in the warm end during compression, and in the cold end during expansion.
The conventional Stirling cryocooler design shown and illustrated in connection with FIGS. 1-1D has been successful in providing cooling under a variety of conditions. However, the Stirling cryocooler design includes two separate moving parts: the first piston 6 and the second piston 8. The complexity offered by these moving parts can offer a disadvantage in extraterrestrial applications such as satellites or space craft, where repair or replacement of worn moving parts is not possible.
Accordingly, efforts have been made to simplify the Stirling cryocooler design shown in FIGS. 1-1D. One such design is the orifice pulse tube cryocooler shown in simplified cross-sectional view in FIG. 2.
Like the Stirling cryocooler shown in FIGS. 1-1D, orifice pulse tube cryocooler 200 includes tube 202 enclosing compressible gas 204 in contact with a moveable piston 206 and first heat exchanger 208 proximate to the compressible gas. Also like the Stirling cryocooler shown in FIGS. 1-1D, orifice pulse tube cryocooler 200 of FIG. 2 includes thermal regenerator 214 in contact with the compressible gas at a point between, first heat exchanger 208 and second heat exchanger 212 in contact with the compressible gas at a point distal from first heat exchanger 208.
Unlike the Stirling cryocooler structure shown in FIGS. 1-1D, however, the orifice pulse the cryocooler 200 has no second moveable piston. Instead, this element has been replaced by pulse tube 220 in fluid communication with tube 202 at the location of the second heat exchanger 212. Pulse tube 220 is in turn in fluid communication with a gas reservoir 222 through an orifice 224. A third, pulse tube heat exchanger 226 is positioned in contact with the gas at the junction between pulse tube 220 and orifice 224.
Operation of the pulse tube orifice cryocooler of FIG. 2 is similar to that of the Stirling cryocooler of FIGS. 1-1D. Specifically, external work is initially applied to piston 206 from an external source. Compressible gas 204 within tube 202 responds to movement of piston 206 first by being compressed, and then by being translated in the direction of the pulse tube 220. Some energy applied to the system at this time is absorbed and dissipated at first (hot) heat exchanger 210.
Translation of the gas compressed by piston 206 is opposed by the constriction offered by orifice 224. Because of the flow resistance posed by the orifice 224, translation of the gas ultimately halts and the gas expands. As a consequence of this gas expansion, the gas cools and second (cold) heat exchanger 212 absorbs thermal energy from the surrounding environment, thereby imparting a cooling effect. Energy is dissipated in the orifice 224 and removed at the (third) pulse tube heat exchanger 226. The pulse tube 220 is an open tube filled with gas that transmits work from the cold end to the orifice, while thermally insulating the cold end from the warm end.
In sum, the cooling cycle of the orifice pulse tube cryocooler shown in FIG. 2 is the same as that of a Stirling cooler, but with the cold piston replaced by passive acoustic component having no moving parts. The pulse tube acts like gas piston, insulating the cold (second) heat exchanger from the warm (third) heat exchanger. The orifice dissipates power at the third, pulse tube heat exchanger, and this dissipated power represents the gross cooling power of the orifice pulse tube cooler.
If the volume of the reservoir is sufficiently large (that is, if it has a large enough compliance, a gas analogy to electrical capacitance), the velocity of gas at the warm end of the pulse tube and the pressure oscillations will be in phase, and the orifice will perform as a gas equivalent to a simple resistor of an analogous electrical system. If, however, the volume of the reservoir is small, the velocity of the gas will lead the pressure of the gas by some phase angle. Optimum cooler performance usually has the gas pressure leading the velocity by about 45° at the second (cold) heat exchanger.
The orifice pulse tube design shown in FIG. 2 offers the advantage of fewer moving parts and reduced complexity over the Stirling cooler. However, the orifice pulse tube cryocooler of FIG. 2 does suffer from certain disadvantages relative to operation of the Stirling cryocooler. Specifically, the gas pressure and velocity are in-phase at the orifice, whereas the optimum condition has the pressure leading the velocity by about 45° at the 30 second (cold) heat exchanger.
Therefore, there is a need in the art for improved cooling structures having simplified designs.